Nanocarrier cancer therapeutics with functional stimuli-responsive mechanisms

Presently, nanocarriers (NCs) have gained huge attention for their structural ability, good biocompatibility, and biodegradability. The development of effective NCs with stimuli-responsive properties has acquired a huge interest among scientists. When developing drug delivery NCs, the fundamental goal is to tackle the delivery-related problems associated with standard chemotherapy and to carry medicines to the intended sites of action while avoiding undesirable side effects. These nanocarriers were able of delivering drugs to tumors through regulating their pH, temperature, enzyme responsiveness. With the use of nanocarriers, chemotherapeutic drugs could be supplied to tumors more accurately that can equally encapsulate and deliver them. Material carriers for chemotherapeutic medicines are discussed in this review keeping in viewpoint of the structural properties and targeting methods that make these carriers more therapeutically effective, in addition to metabolic pathways triggered by drug-loaded NCs. Largely, the development of NCs countering to endogenous and exogenous stimuli in tumor regions and understanding of mechanisms would encourage the progress for tumor therapy and precision diagnosis in future. Graphical Abstract


Where are we today with cancer therapeutics?
Cancers are a group of diseases that are caused by the uncontrolled development of malignant cells, which can infiltrate tissues and spread to other regions of the body. According to the World Health Organization, cancer-related fatalities accounted for nearly 10 million deaths in 2020, and the incidence of cancer is expected to increase rapidly to 28.4 million new cases in 2040 [1]. The most common cancer-related death includes breast, lung, colon and rectum, prostate, skin (nonmelanoma), and stomach cancers. Fortunately, the death rate has been reduced drastically by advances in our understanding of tumor biology and the development of improved diagnostic equipment and therapies.

Current oncological treatments and new therapies: an overview
Several core strategies exist for treating cancer, including surgical intervention, chemotherapy, and radiation therapy, as well as a combination of these techniques. Conventional chemotherapy works primarily by interfering with the genetic material and cell division of cancer cells; however, this approach is non-selective and damages even the healthy cells, thereby resulting in severe side effects and a high mortality rate. In addition, hydrophobic drugs have poor accessibility that reduces the final drug dosage delivered to the tumor tissues, meaning that higher doses must be administered systematically. However, this can lead to severe toxicity in normal tissues and increase the chances of multiple drug resistance (MDR) where cancer cells can evade chemotherapies by developing resistance against cytotoxic drugs immediately after therapy. Therefore, novel drug delivery systems that can enhance specific targeting and reduce adverse side effects in cancer tissues are urgently required. These shortcomings of conventional chemotherapy have prompted the development of smart monitored nanocarrier (NCs)based drug delivery systems that allow targeted drug release at specific sites and reduce toxicity [2][3][4][5] with enhanced penetration [6].
The correlation between drug delivery and nanoparticles (NPs) was first described by Paul Ehrlich [7] using the magic bullet concept, while Speiser et al. [8] were the first to report the regulated sustained release of drugs using a bead polymerization technique. Also, some engineered bioinspired synthetic and cellular systems towards design of nanomedicine platforms for the treatment of cancer [9]. In recent years, an increasing number of studies have investigated tumor biology and reported the construction of NCs using versatile materials, such as inorganic carriers, lipids [10], proteins [11], and polymeric micelles [12,13]. This has led to the development of NC-based drug delivery systems that can deliver chemotherapeutics into the tumor microenvironment on demand. Compared to conventional chemotherapeutics, NCs like liposomes, micelles, and nanoparticles have a variety of advantageous features for use in clinical cancer therapy. For instance, NCs can have a high selective accumulation rate in the tumor microenvironment via the enhanced permeability and retention (EPR) effect [14], which improves treatment efficiency by reducing toxicity in normal tissues. Moreover, active targeted delivery can be achieved using NCs loaded with chemotherapeutic agents and conjugated to molecules that bind to receptors that are overexpressed on cancer cells.
In this review, we highlight various NCs-based drug delivery systems and discuss the targeted mechanisms via which they improve the therapeutic index of chemotherapeutic drugs. In addition, we discuss several endogenous and exogenous stimuli-responsive drug release studies in the context of present-day NCs development, in addition, the metabolic pathways and mechanisms induced by drug-loaded NCs.

NCs used for drug delivery in cancer therapy
Several innovative drug delivery strategies are currently being used to treat cancer, and novel cancer therapies have been developed using an array of nanomaterials, including organic and inorganic particles, and synthetically produced lipids, proteins, and polymers. The delivery of drugs encapsulated in NCs offers several advantages over the direct administration of refined chemotherapeutic drugs. These include enhanced drug delivery, protection of the encapsulated drugs against degradation in the bloodstream, targeted drug delivery, efficient treatment with reduced systemic toxicity, improved drug solubility, and enhanced pharmacodynamic and pharmacokinetic drug properties [13,[15][16][17][18].
NPs have a small diameter of 1-100 nm and a high surface area to volume ratio, which significantly affects their biological activities and allows them to bind, absorb, and transport drugs, DNA, RNA, proteins, and detection molecules. The therapeutic efficacy of NCs are based on the efficiency of nano-drug delivery used in medical treatments which are controlled predominantly by their size, shape, and surface [19]. To date, a remarkable variety of drug delivery NCs have been developed with different sizes, molecules, conformations, and surface physicochemical characteristics (Fig. 1). It is noteworthy that conventional NCs are unspecific, unstable, shows reduced biocompatibility, low permeability, retention effect, drug resistance, and high toxicity due to several barriers encountered during circulation or abnormal vascular networks in tumor microenvironment [20]. However, the targeted stimuli-responsive NCs are precise, shows high stability, biocompatibility, enhanced permeability, shows reduced toxicity and retention effect i.e., exhibit smart behaviors at biological environments. Moreover, in conventional NCs the drug delivery is not targeted and the doesn't show enhanced permeability and retention (EPR) effect to the tumor site, in contrast, delivery of drugs encapsulated in targeted stimuli responsive NCs offers several advantages over the direct administration of refined chemotherapeutic drugs and reduction of side effects that improves therapeutic efficacy [13,15,16].
Here, firstly we discuss the main types of NCs that have been used as drug delivery systems in the treatment of cancer (Tables 1 and 2).

Inorganic NCs
Inorganic NCs have proven to be highly useful in cancer therapy owing to their high surface area to volume ratio and easy conjugation with various cancer drugs. In addition, biocompatibility, low toxicity, and the ability to control drug release have facilitated the use of inorganic NCs, such as mesoporous silica NCs (MSNCs), gold NCs (AuNCs) [21], magnetic NCs (MNCs), carbon nanotube NCs (CNT-NCs), graphene oxide and quantum dots (QDs), as drug carriers (Fig. 2) [22].

Mesoporous silica NCs
MSNCs have a variety of unique characteristics that make them appealing NCs, including a highly porous structure and adsorption capacity, tunable particle size, easy functionalization, excellent biocompatibility, and the potential to act as a physiological container to protect drugs against dysfunction or denaturation [23]. In addition, the pore diameter of some MSNCs can be controlled to regulate drug encapsulation ratios and the release kinetics, allowing anticancer medicines to be delivered in a tailored manner and discharged on time to promote cellular uptake without being released before reaching the intended target site. MSNCs can also adsorb various hydrophobic drugs and thus act as ubiquitous intracellular drug carriers by transporting them across the cell membrane. Moreover, MSNCs can simultaneously adsorb both hydrophilic and hydrophobic drugs for targeted drug delivery. For instance, Chan et al. synthesized phenyl-exMSN-PEG + TA (NTT2_131), which was anchored simultaneously with hydrophilic doxorubicin (DOX) and phobic JM15 anticancer drugs [24]. Moreover, MSNCs have the ability to target the tumor site through both active and passive mechanisms, such as the EPR effect and the evasion of reticuloendothelial system (RES) clearance [25]. Furthermore, the surface of MSNCs can be functionalized with specific ligands, such as hyaluronic acid or transferrin, to target specific tumors or be coated with proteins, enzymes, and magnetic nanoparticles, which function as homing devices. Together, the intracellular absorption, pharmacokinetics, and tissue distribution patterns of MSNCs throughout the targeting process can be significantly altered by regulating the particle size and surface of MSNCs.

Gold nanocarriers
AuNCs have been used as anticancer agents in photothermal therapy (PTT) owing to their excellent ability to convert light into heat [26]. The therapeutic potential of AuNCs is determined by their size, shape, surface plasmon resonance (SPR), and surface biochemistry [27]. The type and functionality of the nanocarrier are controlled by its shape, size, and targeting ligands, leading to high maneuverability and target-specific drug delivery Basically, size and shape of nanoparticles are important parameter that effects cellular uptake and site-specific drug delivery from the synthesized system. It was also noticed that the smaller particles infused into the deep layers of tissues while the 100-200 nm particles stayed on the surface [28], that additionally signifies better bioavailability of small sized gold NPs. The particle size also affect the blood concentration along with circulation time. Moreover, their benign nature and excellent surface functionalization properties make AuNCs effective anticancer drug carriers. The surface functionalization of AuNCs with anticancer drugs can improve their anticancer efficacy via the combined effect of the nanoparticledrug conjugate on tumor cells, while anchoring with a specific antibody can improve their target specificity [29]. Furthermore, the surface modification of Au NPs with a pH-sensitive material can allow them to specifically target tumor cells, which have a lower pH than normal cells. Chaudhari et al. [30] investigated the metabolic pathways and mechanisms induced by methotrexate (MTX)-AuNC in breast cancer cells. MTX-AuNC targeting the folate receptor, demonstrated considerable uptake by the breast cancer cells, as well as substantial down-regulation of the anti-apoptotic gene and up-regulation of pro-apoptotic genes [30].

Magnetic NCs
MNCs have recently received a lot of interest owing to their immense potential as heat regulators for treating hyperthermia as well as their ability to transport drugs in a targeted manner, which can reduce systemic effects [43]. In addition, MNC-based cancer therapies have novel features that are not observed in traditional approaches, such as minimal toxicity and controlled drug release. The behavior of MNCs is predominantly affected by their hydrodynamic diameter, surface chemistry, and magnetic properties. Surface chemistry is crucial for preventing clearance by the RES and improving halflife within the blood circulation. For example, researchers have modified the surface of MNCs with neutral and hydrophilic ligands, such as PEG or P(S/V-COOH) polymers, and reported an improvement in their circulatory half-life from minutes to months [44]. Anchoring the surfaces of MNCs with new ligands has facilitated targeted In vivo application [42] drug delivery, and the anchoring of single or multiple drugs on MNCs has recently been studied to improve anticancer drug efficacy. The stability and biocompatibility of MNCs can be optimized by anchoring them with organic or inorganic compounds, which can improve the efficacy of anti-cancer chemotherapy and gene therapy [45]. It is worth to mention that MNCs, offers a suitable tool for cancer treatment and diagnosis due to their unique features which distinguish them from other NCs. These can act through magnetic drug targeting, making them therapy magnetically responsive, therefore it can be controlled inside the human body by external magnets, and eventually absorbed into tumor tissues. Their utilization in magnetic resonance imaging (MRI) delivers a high divergence for generating the most detailed imaging. Such as, magnetic iron oxide NPs widely applied for lung MRI due to their good magnetization and biocompatibility as well as their appropriate drug uptake and subsequent release [46]. Similarly, water-dispersible PEIconjugated iron oxide NPs have been also exploited for MRI-based cancer imaging [47].

Carbon nanotube NCs
Carbon nanotubes (CNTs) are long cylindrical structures with flexible NCs characteristics that have sparked interest as drug delivery molecules owing to their unique biological, physical, and chemical capabilities [48]. Indeed, CNTs have been used to deliver anticancer drugs, such as DOX, paclitaxel, methotrexate, and small interfering RNAs (siRNAs), to treat a range of malignancies. The developing siRNA technology merged with chemotherapy has presented significant rationale in cancer therapies. The main challenges in the fabrication of siRNA/chemotherapeutic drug co-loaded NPs are specific targeted delivery, sufficient cargo protection, and site-specific release [49]. CNTs are a popular multifunctional drug delivery system owing to their large surface area, high adsorption capacity, durability, ease of functionalization, an exceptionally high lengthto-diameter ratio, and excellent intracellular absorption [50]. In addition, the surface characteristics of CNTs can be readily altered using covalent/noncovalent crosslinking, making them a favorable nanomaterial. Self-assembly Aptamer-tethered DNA assembly Stronger targeting ability, higher cellular uptake Cancer cell imaging [108] Self-assembly DNA-aptamer conjugated RNA-triple helix hydrogel Efficient cellular uptake and enhanced nuclease resistance with superior biocompatibility Triple negative breast cancer detection and treatment [109] Self-assembly Folate-modified MPEG-PCL Improved bioavailability, low toxicity, sustained drug release Colorectal cancer mice model [110] Self-assembly Folate receptor-targeted β-cyclodextrin (β-CD) Biosafety, bioavailability, and improve curcumin drug loading capacity Cervical cancer, fibroblast cells [111] The hydrophobicity of CNTs leads to π-π stacking interactions with a variety of drugs and medicinal compounds, including those with an aromatic ring such as anthracyclines; however, CNTs have a low solubility that makes them ineffective drug delivery molecules. The solubility of CNTs can be improved by surface functionalization with various molecules using covalent/noncovalent bonding and electrostatic forces to make them more hydrophilic, thus altering their biocompatibility profile [51,52].

Quantum dots NCs (0-D)
Quantum dots (QDs) s are uniform spherical (0-D) NCs with very small sizes of 1-10 nm [53]. These fluorescent particles have exceptional physio-chemical characteristics, including a very large surface area, biocompatibility, highly customizable photoluminescence, strong signal brightness, and high photostability. These features have inspired researchers to use QDs as potential NCs for targeted and traceable drug delivery systems, to monitor intracellular processes in real time, and for in vivo molecular imaging. However, the hydrophobicity, easy agglomeration, and high adsorption affinity of QDs for the surrounding impurities make them poor candidates for therapeutic applications. Fortunately, these shortcomings can be avoided by coating QDs with ionic species or layering them with ligand shells [54]. For instance, imaging probes, smaller hydrophobic or hydrophilic drugs, and targeting agents can be embedded between the inorganic core and the amphiphilic polymer coating layer [55,56].

2D-materials NCs
2D materials have excessive light photodynamic and heat conversion proficiency, therefore they retain various benefits in biomedical applications. These characteristics provide them high potentials in medicine fields, including imaging [57], sensing [58] and therapy [59,60].
In the meantime, 2D materials exhibited a challenging capability in drug delivery with various advantages. One of the exclusive crucial features of 2D materials is the lamella organization, that offers the huge surface space for high drug loading efficiency [61]. Yu et al. synthesized a reduced GO nanocomposite modified with a polydopamine (PDA). They coated the dopamine-modified rGO surface with antiarrhythmic peptide 10 which limit tumor development about more than 95% while used with radiotherapy [62]. This innovative drugs possesses several abilities, such as large surface area, excellent biocompatibility, and a high drug loading capacity. Moreover, these NCs confirmed subsequent pH responsiveness and drug release. As stated by Xing et al. injectable hydrogel made of black phosphorous nanosheets and cellulose exhibits noteworthy antitumor activity in contrast to PTT. Interestingly, these nanoscale hydrogel platform is non-toxic and 100% biocompatible as confirmed by both in vitro and in vivo studies [63].

Organic nanocarriers
Organic NCs have been studied for decades and contain a diverse range of components. They are biocompatible, have low drug loading capacity and the drug molecules are encapsulated or conjugated.

Liposomes NCs
Liposomes are made up of an exterior lipid layer and a core that contains either hydrophobic or hydrophilic medications; they were the first nano-scale drugs to be licensed for clinical use [64]. Liposomes can be modified to perform a variety of tasks by altering the lipid layer structure; for instance, the lipid layer can be altered to mimic the biophysical properties of living cells [65], which can improve the efficiency of targeted drug delivery [66]. The inner aqueous core of liposomes can be loaded with amphiphilic drugs using various techniques, such as the ammonium sulfate gradient method [67]. However, standard liposomes are rapidly eliminated from the bloodstream and can be detected by opsonin proteins, leading to phagocytosis and destruction by macrophages. To overcome this issue, surface-modified liposomes have been developed that can be grafted with monoclonal antibodies, glycoproteins, carbohydrates, vitamins, antibodies, and peptides to actively target cancer cells (Fig. 3). Similarly, Najlah et al. developed stealth liposomes by modifying the surface coating of a hydrophilic lipid polymer derivative of PEG [68]. This modification was shown to improve the efficacy of the encapsulated drugs by prolonging their circulation in the blood and reducing their elimination [69].Unfortunately, the reliability of the PEGylated liposomes decreases upon systemic injection, which may prolong their circulation in the blood; therefore, novel PEG-dendron phospholipids have been developed to produce super-stealth liposomes. In vitro studies have shown that ligand-targeted liposomes can promote the internalization of liposome-drug conjugates via prostate-specific membrane antigens on the targeted cell [10]. Decades of liposome research have facilitated the development of liposomes that are a suitable platform for the in vivo administration of various anti-cancer drugs, including docetaxel [70], and nucleic acids [71]. Indeed, liposomes are now increasingly being used to treat breast [72] and prostate [73,74] cancers. In addition, more liposome-based medicines are currently in clinical trials for cancer therapy [75,76].

Protein-based NCs
Proteins are essential macromolecules that possess unique functions and characteristics in biological materials and manufacturing field; therefore, they are utilized as a starting material for the synthesis of NCs (Fig. 4) [77,78].
Since protein-based NCs display good biodegradability and low toxicity, they are used to carry both pharmaceuticals and nutraceuticals [79]. Additionally, protein NCs have high stability, are non-antigenic, can be metabolized, and it is easy to synthesize their particles, modify their surface, and monitor their size [11]. Owing to their promising characteristics, protein NCs have been used for a variety of targeted uses, such as cancer therapy [80,81], pulmonary delivery [82], and vaccine delivery [83]. Protein NCs can be incorporated into biodegradable polymers in a monolithic particle structure, such as a microsphere [84], to allow controlled and sustained drug release and can be produced using proteinaceous Schematic representation of polymeric micelles. Self-assembly of di-block copolymers into a polymeric micelle takes place above the critical aggregation concentration. The hydrophobic drug is encapsulated into the hydrophobic core. Figure has been created using Biorender materials such as silk protein fibroin [85], bovine [86], human serum albumin [87], and gliadin [88].

Micelles NCs
Micelles are spherical nanosized structures that are formed via the self-assembly of amphiphilic block copolymers that have both hydrophilic and hydrophobic portions in aqueous solution, thus forming a hydrophobic core and a hydrophilic shell, known as polymeric micelle (Fig. 5) [89,90]. Micelles can be formed intrinsically at specific concentrations (critical micelle concentration (CMC) and temperatures. For instance, if the solvent portion is hydrophilic and its concentration exceeds the CMC, the polar region of the co-polymer is drawn toward the solvent, whereas the hydrophobic region is repelled from the solvent [12]. In polymeric micelles, the hydrophobic core serves as a reservoir site for the incorporation of hydrophobic drugs, while the hydrophilic shell stabilizes the core and keeps the polymer and drug intact, making micelles a suitable candidate for intravenous administration. Genexol-PM [PEG-poly(D,Llactide)-paclitaxel] was the first cremophor-free polymeric micelle to contain paclitaxel and was shown to have no adverse reactions and a good toxicity profile in advanced refractory cancers [91].

Self-assembled drug NCs
The spontaneous organization of molecules into ordered geometric structures is known as molecular self-assembly and provides both inorganic and organic structures with unique qualities via non-covalent interactions. The spontaneity, versatility, inexpensiveness, and simplicity of selfassembly mean that it is extensively used for designing nanoscale biomaterials [92] and has various biomedical applications, including drug delivery, tissue engineering, and regenerative medicine. Self-assembled NCs such as micelles, polymeric NPs, liposomes, and CNTs have been shown to improve drug delivery, prolong blood circulation, control drug release kinetics, impart molecular targeting, improve tumor accumulation [93] and bioavailability, allow proper encapsulation, and protect the drug from the external milieu [94]. Ultimately, these selfassembled NCs help to overcome physiological barriers in vivo during drug delivery.

Supramolecules
Supramolecules are molecular assemblies that are held together by electrostatic interactions, metal coordination, hydrophobic attractions, host-guest interactions, and van der Waals forces [95]. Also, supramolecular compounds are easy to realize the "Lego-like" construction of NCs, which have excellent editability. These interactions provide the NCs with stability in body fluids and improve their characteristics, including durability, sustained release, and transport efficiency. Accordingly, supramolecules are used as drug carriers for targeted anticancer delivery systems [96]. Indeed, the side effects of anticancer drugs, such as DOX, on normal cells can be controlled by using amphiphiles to generate supramolecular aggregates for cancer therapy [97].DOX-anchored supramolecular polymersomes circulate in the bloodstream for a longer time, and in vivo experiments have demonstrated their superior antitumor efficacy against malignant HeLa cells with decreased cytotoxicity [98].

Stimuli-responsive NCs drug release
In recent decades, the development of novel polymers has prompted the study of smart stimuli-responsive drug delivery systems [112,113]. Wei et al. proposed that stimuli-responsive nanomedicines determine noteworthy benefits via their adaptive transition application during drug delivery cascade [114]. A number of literature suggests that stimuli-responsive materials improve their pre-designed functions in response to the tumor microenvironment (TME) or intracellular signals, for instance conversion of the surface charge, PEG deshielding, exposure of tumor-targeting ligand, and controlled drug release [115][116][117]. It is broadly known that the TME has distinctive physiological characteristics such as acidic pH [118], hypoxia [119], and induction of certain enzymes [120]. Overall, the NCs might response to exogenous stimuli, including temperature, magnetic field, light etc., and endogenous stimuli, including pH, ATP, H 2 O 2 , enzyme, redox-potential, and hypoxia etc., although the stimuli might be emerged in TME or inside cancer cells (Fig. 6) [121].

Endogenous stimuli
Also known as an intrinsic stimuli, endogenous stimuli include changes in the internal pH levels, enzyme activity, redox activity, hypoxic conditions of the body.

pH-responsive NCs
Tumor cells primarily generate energy by enhancing glycolysis and then fermenting lactic acid in the cytosol, which is known as the Warburg effect [122]. The increased acid production results in an acidic pH in and around the cancer cells compared to that in normal cells, which is why pH has been established as an effective physiological property for targeted and controllable drug delivery by pH-responsive NCs [123,124]. To achieve targeted and site-specific drug delivery, pH-responsive NCs exploit pH gradients in the physiological milieu [125]. The normal physiological pH is neutral (7.0-7.4), whereas the pH of the tumor microenvironment is highly acidic due to high metabolic activity and inadequate perfusion [126]. The main feature of pH-triggered NCs is their transition from hydrophobic to hydrophilic in response to acidic pH levels, which can alter their size or behavior. For instance, Tang et al. reported pH-sensitive NC systems based on reversibly ionizable carboxylic groups that undergo protonation and deprotonation due to changes in pH, leading to NC swelling, rupture, and drug release [127]. Several pH-responsive materials have been used for controlled drug delivery, including curcumin-loaded pH-sensitive N-naphthyl-N,O-succinyl chitosan polymeric micelles that were developed for colon-targeted drug delivery and exhibited pH-dependent release kinetics in colorectal cancer cells [128]. In addition, Ishida et al. [69] synthesized various pH-sensitive sterically-stabilized liposomes composed of DOPE/HSPC/CHEMS/CHOL/mPEG 2000-DSPE. These liposomes enhanced drug retention and pH-sensitivity, increased the blood retention time of encapsulated DOX, and targeted antigens on cancer cells to allow controlled drug release in the acidic tumor microenvironment [69]. Unfortunately, tumor cells can still become resistant to anticancer drugs delivered by NCs, thus compromising their therapeutic efficacy. To address this issue, Guo et al. [129] developed a novel dual pH-sensitive micelle NC, PEO-b-P(DMAEMAco-MAEBA) combined with a specific inhibitor (A01) of the TMEM16A ion channel, which is overexpressed in lung adenocarcinoma tissue. These micelles released A01 into the surrounding weakly acidic microenvironment of lung adenocarcinoma cells but were also internalized by the cells and entered their endosome-lysosome compartments, where they inhibited TMEM16A and MAPK signaling. pH-sensitive self-assembled drug carriers have also been extensively used. For instance, Song et al. synthesized biodegradable pH-responsive PEGylated DOX micelles that self-assemble in aqueous solutions via esterification and Schiff base reactions [130]. These NCs had a high drug payload and an excellent pH-responsive controlled drug release profile, as well as rapid prodrug internalization and enhanced antitumor activity against MCF-7 cancer cells. Interestingly, Chen et al. prepared a nature-inspired pH-responsive organic-inorganic hybrid capped mesoporous silica NPs based on biomineralization mechanism having low toxicity, enhanced cellular uptake and efficient drug release [131]. In another study, a group of researchers established a cRGD-decorated pHresponsive polyion complex micelle for DOX intracellular targeted delivery to enhance tumor inhibition and decrease cytotoxicity [132].

Redox-responsive NCs
Glutathione sulfhydryl (GSH) is a tripeptide compound in which glutamic acid is conjugated through its side chain to the N-terminus of cysteinylglycine. Since GSH levels are fourfold higher in cancer cells than in normal cells [133], it is used to produce redox-responsive NCs. In particular, GSH cleaves the disulfide bond of NCs in cancer cells, leading to controlled redox-driven drug release [134].
Various redox-responsive drug delivery systems have been reported in recent years, including liposome, micelle, nanogel, and prodrug-based systems. Redox-responsive liposomes initiate drug release in the presence of GSH (reducing agents) via disulfide linker reduction or liposome membrane destabilization. For instance, Chi et al. [135] synthesized a novel Chol-SS-mPEG/HA liposome (conjugated with PEG via disulfide linkages) that displayed cytoplasmic drug release triggered by GSH due to PEG de-protection and liposome agglomeration. In another study, Yin et al. [136] prepared chitooligosaccharides (COS) conjugated with cholesterol through a disulfide linker, wherein linker reduction caused COS removal, liposome instability, and rapid drug release. Micelle-based drug delivery systems are often used because of their ease of administration, enhanced circulation time when PEGylated, high structural stability, good hydrophobic drug encapsulation, and ability to respond to the external environment [137]. Micelles assembled using disulfide linkages are redox-responsive, as they can be cleaved in the presence of GSH (or a reductive environment). Sun et al. prepared redoxresponsive amphiphilic glycan conjugates by linking heparosan with deoxycholic acid via disulfide bonds [138]. When the micelles reached cancer cell cytosol, they rapidly disassembled and released the drug. Similarly, Kang et al. [139] developed a redox-responsive system by linking gambogic acid with poly(amido amine)s via amide bonds, which self-assembled into micelles. The presence of disulfide bonds in a reductive environment led to rapid disassembly and release of the encapsulated drugs.
Although disulfide bonds are the main component when designing redox-responsive NCs, diselenide bonds can be used to prepare mesoporous silica-based NPs [140]. For instance, controlled drug release can be achieved using albumin or myoglobin by conjugating them to mesoporous silica-based NPs via diselenide bonds, which yields an NC that is highly responsive to GSH and H 2 O 2 [141]. In addition, Lei et al. developed a novel intrinsic redox-responsive metal-organic framework carrier using iron, aluminum, or zirconium as metal nodes and 4,4-dithiobisbenzoic acid (4,4-DTBA) as the organic ligand [142].

Enzyme-responsive NCs
Upregulation of several enzymes are linked to the pathophysiology of many diseases, including infection, inflammation, and cancer. Since most of the enzymes are present in similar concentrations in both cancerous and normal cells [143], these NCs cannot be used for intracellular drug release; however, enzymecleavable peptides can be used to generate enzymetriggered NC deshielding, which can ultimately allow the drug to be released. Enzyme-triggered NCs can be prepared by modifying the NC surface, which can respond to the biocatalytic reaction of enzymes that are overexpressed in the extracellular microenvironment of cancer cells. Such as, matrix metalloproteinases (MMPs) and hyaluronidases (HAs) are the extracellular enzymes that are mostly upregulated in tumors [144,145]. The upregulation of these extracellular enzymes can be used to trigger NCs for reduction of their size and surface ligand exposure for tumor infiltration. It is well-stated that cathepsin and legumain showed overexpression in several cancer cells [157][158][159][160][161]. Legumain is the endopeptidase which specifically cleaves linkers containing asparagine or aspartic acid residues which are upregulated in the various cancer cells lysosomal compartments. For instance, Liu et al. [162] developed a legumain cleavable liposome consisting of alanine-alanine-asparagine substrate linked to cell penetrating peptides (trans-activating factor) carrying doxorubicin. In another study, Cai et al. [146] developed an enzyme-responsive colon-specific delivery system based on hollow mesoporous silica spheres conjugated with biodegradable chitosan via cleavable azo bonds and encapsulating DOX. This NCs system displayed enzyme-responsive drug release in the presence of colonic enzymes.

Hypoxia-responsive NCs
In solid tumors, the poor vascularization system is possibly to form low oxygen level (hypoxia), that plays a critical role in cancer progression, including distant metastasis [147]. Consequently, many approaches have been employed for treatment of hypoxic tumors, primarily including rising the oxygen level and by means of hypoxia activatable drugs, etc. [119]. Till date, different types of nanocarriers have been designed for the drug delivery system to hypoxic tumors [148], such as silica nanoparticles [149], liposomes [150], layer-by-layer nanoparticles [151], polymeric micelles [152], nanovesicles [153], polymersomes [154], and albumin nanoparticles [155] (Table 3).

Tumor-metabolite responsive NCs
In tumor cells, metabolic reprogramming changes the metabolic pathways to encounter their unquenchable craving for nutrient and energy. In this regard, tumorassociated metabolites are generally valued in estimating tumor incidence and advancement timely. Insufficient nutrient supply fluctuates cancer energy metabolism, in this case lactic acid (extracellular metabolite) declines the pH of the tumor microenvironment. Fruehauf et al. mentioned that the oxamate-functionalized NPs proficiently sequestered lactate dehydrogenase (LDH) to make an OxNP-protein complex. This effort validates a great concept for tuning NPs sensitivity via conjugation with  MR imaging of solid tumors, hypoxia and metastasis a fundamental protein to aim a particular metabolite of cancer disease [156].

Exogenous stimuli
When trigger stimuli are caused by external factors, such as a change in temperature, ultrasound, or light responsiveness, they are known as exogenous stimuli. In this system, contrast agents are employed to visualize NCs retention in cancer cells.

Temperature responsive NCs
Thermo responsive NCs use temperature changes as the trigger to release their cargo, as they can change their hydrophilic and hydrophobic balance, solubility, or structural features in response to a particular temperature. These NCs show a low critical solution temperature (LCST), below which the constituents of a mixture are misciblee [157]. Thermoresponsive polymers with an LCST are promising candidates for drug delivery, as are stimuli-responsive hydrogels that can prevent drug degradation and display rapid on/off switching [158,159]. Hydrogels can be injected as viscous liquids and become jellified under physiological conditions. For instance, Tipa et al. [160] synthesized a biocompatible and injectable thermo-responsive hydrogel that was jellified in situ at 28 °C and injected with a 5 N force. The hydrogel contained clay NPs whose interlaminar spaces were intercalated with methylene blue (MB) for controlled release with enhanced encapsulation efficiency. Notably, the Pluronic hydrogel demonstrated controlled long-term MB release. Similarly, Ahsan et al. [161] formulated a thermosensitive chitosan-based, cross-linked injectable hydrogel for the sustained delivery of disulfiram to cancer cells as a long-term cancer therapy, and found that the drugloaded hydrogels allowed greater cellular uptake than the free drugs. Graphene oxide (GO) has a high ratio of surficial functional groups that permits the modification of bioactive molecules, as well has high dispersibility in aqueous conditions; however, its applications are limited by non-targeted and uncontrolled drug release. To overcome these limitations, Kazempour et al. modified GO with biodegradable and hydrophilic polymeric components, such as poly(Nvinylcaprolactam) (PNVCL) and poly(glycolic acid), which conferred pH and temperature sensitivity to allow the effective loading and release of oxaliplatin in MCF-7 cells [162]. Similarly, Farjadian et al. synthesized smart temperatureand pH-responsive NCs based on a random copolymer of poly(N-isopropylacrylamide-co-acrylamide) [163]. The presence of a hydrophilic group in acrylamide increased the LCST to 37 °C (close to body temperature) and ultimately made the structure suitable for drug delivery. This structure was further modified with hydrazine and conjugated to DOX via a Schiff base linkage (acid-cleavable bond) to make NCs responsive to heat and pH.
Given that hyperthermia increases the temperature of body tissues to as high as 113 °C and increases the sensitivity of cancer cells to chemotherapeutics, combining these two effects as thermo-chemotherapy could enhance the efficacy of cancer therapy. Mirrahimi et al. [164] developed a novel multifunctional nanocomplex, consisting of an alginate nanogel loaded with cisplatin and AuNPs, for combined therapy. In vivo thermometry studies showed that under 532 nm laser irradiation, the temperature increased and exerted a higher thermal dose due to the optical absorption characteristics of AuNPs; this finding indicated that these NCs suppressed tumor growth due to their thermo-responsive behavior.

Light-responsive NCs
Various NCs can respond to light and have the potential to adjust the irradiation wavelength, power, and affecting area [165]; therefore, light-responsive NCs have recently been developed for targeted controlled drug delivery. NCs can be sensitive to any form of light, including ultraviolet, visible, or near-infrared, which can also affect biological systems such as cancer cells or tumors. By controlling the range of irradiation, light-responsive tumor therapies can be precisely conducted to minimize or avoid negative effects on normal cells. Polyplexes [166], polymeric micelles [167], liposomes [168], nanogels [169], and nanorods [170] have all been exploited as light-responsive NCs using materials such as gold nanocomposites, CNTs, graphene, and organic molecules.
NCs can undergo conformational changes in response to light via the structural conversion of light-sensitive molecules. For instance, UV-Vis-responsive photoswitching NCs have exhibited potential for loading with drugs, including DOX, docetaxel, and paclitaxel, for cancer therapy [171]. However, UV-Vis has a short wavelength, which limits the application of these NCs. Therefore, NIR lightresponsive NCs may be more appropriate for controlled drug delivery [172]. Fan et al. [173] synthesized photoresponsive degradable hollow mesoporous organo silica NCs for anti-cancer drug delivery. The NCs were based on singlet oxygen ( 1 O 2 )-responsive bridged organoalkoxysilanes [from a 9, 10-dialkoxy-anthracene (DN)-based precursor] and wrapped with graphene oxide QDs. Upon irradiation, the these QDs generated 1 O 2 , leading to the cleavage of 1 O 2 -responsive bridges, NCs degradation, and the release of the anticancer drug.

Ultrasound-responsive NCs
High-frequency waves, named as ultrasound might affect NCs for the controlled drug release at tumor sites. Currently, it has been considerably applied in the various biomedical applications, for instance gene delivery and imaging-guided drugs. It can initiate the release of the drugs via the heat produced from cavitation phenomena [174]. The ultrasound-responsive nanocarriers might be used for tumor ultrasound imaging, which is considerably safe, cost-effective and commonly applied in clinics. The gas and perfluoropentane incorporated NCs [175], which could generate gas (e.g., CO 2 ) in biological atmosphere [176], have exhibited tumor-site specific imaging at great intensity. Besides, the ultrasound-responsive property might be used for boosting the intracellular delivery of siRNA or DNA in tumors [177]. Nevertheless, the huge size of ultrasound-responsive NCs can limit their penetration within tumor tissues, due to the weak penetration [178]. In general, the drug-loaded ultrasoundsensitive NCs might be used for tumor therapy [179] as well as theranostics [180].

Mechanisms of targeting stimuli responsive NCs to tumor site
Cancer cell specificity is an important feature of NCs for drug delivery because it improves their therapeutic efficacy while sparing normal cells from damage. However, NCs can only reach cancer cells if they deceive the body's own defense system. There are two mechanisms of targeted drug delivery: passive and active targeting (Fig. 7) [181]. Both of these methods aim to prevent contact between normal cells and harmful chemicals, reduce negative dose-limiting consequences, and prevent the development of drug-resistant malignant cells.

Passive targeting
Compared to normal cells, rapidly developing malignant cells put higher stress on the endothelium of blood vessels, resulting in the formation of new vessels or the depletion of existing vessels. During this process, pores are formed on cancer cells, allowing the adsorbed macromolecules or NCs to enter the newly formed vessels [182]. Inadequate lymphatic drainage within tumors prevents the NCs from leaving, thereby enabling them to discharge the loaded drugs into the cancer cells. This phenomenon is known as the EPR effect and is an important factor for passive targeting [125]. This effect is influenced by the diameter of NPs; studies have shown that smaller NPs have a higher permeation but do not spill into regular channels [183], whereas micro molecules are preferentially removed by immune defense systems [184]. Interstitial fluid pressure (IFP) is another obstacle to the effective deposition of drug-loaded nanocarriers in solid tumors [185]; however, successful NCs alterations can bypass biological obstacles. At first, passively targeted NCs moved the clinic several years back with the agreement of PEGylated liposomal doxorubicin (DOXIL ™ ) [186]. There are numerous studies regarding passive targeting by lipid NPs also. For instance, sclareol-SLNs with an typical particle size of 88 nm, has disclosed considerably growth inhibition effect on lung cancer cells along with a sustained drug release when compared to the free drug alone [187]. Conjugation of curcumin with solid lipid NPs is additional report for passive targeting of osteocarcoma cancer tissues with extremely higher tissue availability [188]. In contrast, for passive targeting of melanoma and glioblastoma, temozolomide-SLN exhibited maximum inhibition in proliferation with low toxicity in normal counterparts when compared to temozolomide without solid lipid NPs [189].
A variety of NPs are now being used in clinical trials, including Genexol-PM ™ in Korea and ProLindac ™ and Opaxio ™ in the United States [190]. Awada et al. and Burris et al. have also validated the standards and/ or therapeutic efficacy of a variety of other NCs in clinical trials, including AZD2811, NK911, and CPX-1 [191,192]. Though, many of these NCs basically change the toxicological profile, or drug solubility, some have also presented substantial survival advantages and improved therapeutic efficacy as stated in clinical reports. One case is Abraxane ™ which established notably greater response rates as compared to standard paclitaxel treatment in a phase III trial conducted in breast cancer patients with metastasis [193].This findings is tremendously promising for this field which opens up innovative prospects in employing NCs as smart delivery vehicles for various drugs. Unfortunately, passive targeting has several drawbacks, including non-specific drug distribution, restriction of the EPR effect, and varying blood vessel permeability across tumors [194].

Active targeting
Active targeting approaches are considerably more complicated than the passive approaches. It is remarkable that the active targeting is critical for the drug and gene delivery to the site of interest and thereby augments the therapeutic efficacy and restricts side effects on the normal tissues. Active targeting is able to improve the amount of drug delivered to the target tumor cell compared passively targeted NCs. Following accumulation in the tumor site, the drug competence may be even augmented by the active targeting. This is accomplished via the decoration of the NCs surfaces with ligands which could bind to overexpressed receptors in tumor cells [181]. This strategies will expand the affinities of the NCs for the tumor cell surface to improve the drug penetration capability. To ensure that the drugs are released at a specific location at a predetermined ratio, the binding of NCs ligands to cancer cell receptors causes receptor-mediated endocytosis, which allows the NCs to effectively deliver the therapeutic drugs. Consequently, active targeting is specifically tailored toward the transport of macromolecular drugs, such as proteins and siRNAs. Targeting ligands include proteins (antibodies and fragments), nucleic acids (aptamers), and other peptides, vitamins, and carbohydrates [195]. The ligands attach to receptors on targeted cells, among which the most common are the transferrin receptor, folate receptor, glycoproteins, and epidermal growth factor receptor. Bartlett et al. demonstrated that the active targeting of nucleic acids into cells may also be used to silence luciferase beacons targeting the transferrin receptor in neuroblastoma xenografts [196]. Although there are currently no commercially available active targeting NCs on the market, clinical trials are under way for NCs based on liposome-targeted and polymeric NPs. For instance, MBP-426, MCC-465, SGT53, MM-302, BIND-014, CALEA-01, cetuximaddecorated Doxil/Caelyx liposomes, and a retroviral vector are currently in phase I/II clinical studies for primary therapeutic targets, including EGF, Tf-R, PSMA, the surface of gastric cancer cells, and HER-2 [191,192].
In general, crossing the tumor endothelium is a fundamental footstep in the expedition of NPs from the administration region to diseased tissues. Earlier literature proposed that the vascular barrier of tumor blood vessels is compromised [197]. It is believed that compared to healthy blood vessels, tumor blood vessels could demonstrate certain pathological features that affect NPs Fig. 7 Mechanisms of targeting NCs to tumors. A Passive targeting by nanomedicines is due to the enhanced permeability and retention (EPR) effect, which involves their extravasation from leaky tumor vasculature and poor lymphatic drainage. B Active targeting is achieved by functionalizing nanomedicines with targeting ligands that recognize tumor cell receptors, which increases cell specificity and uptake. Figure has been created using Biorender activity throughout accumulation inside tumor tissues [198] concisely, systemically controlled NPs could transport from the tumor blood vessel lumen via the endothelial spaces into the tumor interstitial area [197]. This is the central mechanism for both active and passive NPs targeting [199].

Challenges and future prospects
As nanotechnology and innovative functional materials have advanced, so has interest in polymer and hybridbased nanocarriers for drug delivery. Developing nanocarriers for the delivery of pharmaceuticals has long been a focus of many scientists. Despite substantial advances with hybrid nanocarriers, polymer-based nanocarriers have had a lot of success in clinical studies. While in circulation in the blood, nanocarriers must (1) prevent early leakage of therapeutic agents, (2) possess targeting ability so that therapeutic agents may aggregate at tumor sites and decrease severe side effects on healthy tissues, show biocompatibility, and (3) demonstrate degradability.
There has been a surge in interest in nanocarriers due to a variety of factors including their biodegradability, biocompatibility, physiological medium stability, and structural instability in malignancies. Polymer or hybridbased nanocarriers with stimuli-responsive properties have required a great deal of effort to create effectively. They were able to change their pH, enzyme, thermal, and ultrasonic responsiveness to deliver medications to tumors. It is possible to deliver chemotherapy chemicals to tumors more accurately owing to nanocarriers, which both encapsulate and distribute the chemicals in a targeted manner. To improve the therapeutic efficacy of hybrid-based nanocarriers, these nanocarriers have also been used in imaging. Hybrid nanocarriers, which are being developed at a rapid pace, are anticipated to stimulate a combination of diagnostic and therapeutic in the area of cancer therapy. Researchers have proven that theragnostic hybrid nanocarriers with theragnostic capabilities may be utilized to correctly cure cancer in the future. To date, the hybrid-based nanocarriers' therapeutic efficacy has been improved by using these nanocarriers in imaging. Nanocarriers that combine diagnostic and therapeutic capabilities are anticipated to be encouraged by the rapid development of hybrid nanocarriers. Hybrid nanocarriers with theragnostic capabilities may be employed to correctly treat cancer in the future as previously predicted.
A few issues remain in the design of nanocarriers containing stimuli-responsive polymer or hybrid materials. Although progress has been made in the development of stable nanocarriers in a healthy medium and stable nanocarriers at tumor locations, it is still challenging to accomplish. The discovery of novel hybrid-based nanocarriers, despite significant progress, continues to impede the development of nanomedicines. Nanocarriers have also been overdesigned in recent decades to integrate many functionalities into a single molecule in order to create a multifunctional nanomedicine. Nanocarriers that are too complicated to be used in clinical trials are generally the result of over-engineering. Nature is always guided by the simplest and most cost-effective rules of operation. Polymer or hybrid nanocarriers that can be synthesized economically and at large scale will thus be the most beneficial. In the end, these issues and obstacles will encourage biomedical scientists to better decipher the relationships between the nanocarriers structural and functional aspects and to enhance nanocarrier design. Consequently, experts from a variety of fields will be better able to realize the promise of stimuliresponsive nanocarriers for cancer treatment if they work together.

Conclusion
In this review, we have provided a broad overview of various materials that can be used as drug delivery NCs for cancer therapy, as well as stimuli-based drug delivery systems and the different mechanisms of targeting. Due to their unique characteristics, clinicians have been able to use drug delivery NCs as monotherapies or as adjuncts to current treatments in order to enhance therapeutic efficacy (Table 4). Although some of these systems have failed in clinical trials, numerous new and intriguing materials are currently under development and display tremendous potential, indicating that new therapeutic alternatives may soon be available. In particular, stimulisensitive NCs provide high specificity as well as various desirable drug delivery functions, including regulated release, tumor accumulation, and better diagnostic and therapeutic efficacy.